Asbestos fiber is used extensively in a number of common products such as asbestos-cement boards, sheets, shingles, pipe, paper, millboard, etc., which are produced by the so-called "wet" or "dry" processes. Both processes include filtration in which a liquid, typically water, is withdrawn from within a mass or body containing asbestos fibers.
The wet process conventionally involves the steps of forming an aqueous slurry of asbestos fibers, Portland cement, silica, and other minerals, fillers, binders, and fibers in water, flowing the slurry onto a filter element upon which the dispersed solids of the slurry may be collected, removing water in the slurry through the filter element by filtration, with or without accompanying pressure forming, and then removing the formed mass of asbestos fibers and cement from the filter element. Because the asbestos fibers and the finely divided cement particles inhibit filtration, there are very practical limitations upon the thickness of the resulting layers which can be collected on filter elements in this fashion. Thus, as the collected mass builds up on the filter element, the rate of filtration rapidly decreases, making it impractical to form structures of great thickness with a rate of through-put which must be obtained in commercial operations.
In the conventional dry process used to form asbestos-cement articles, the dry materials, such as asbestos, silica, cement, and pigment, are blended together and formed into batches. Sufficient water is added to the batch to form a plastic paste which can be molded, calendered, pressed, or extruded prior to standard asbestos-cement curing operations.
The filtration properties of chrysotile asbestos fibers differ substantially with the source or mine location, as well as by type, grade, and consistency, ranging from a relatively fast filtering "harsh" fiber to a slow filtering "soft" fiber. Typical of the latter class of fibers are chrysotile asbestos materials found in the Quebec, Canada area. The filtration characteristics of chrysotile asbestos are a significant consideration in determining the suitability or utility of a particular chrysotile fiber or fiber blend in wet process and dry process manufacturing procedures.
A large number of treatment methods have previously been proposed and evaluated for the purpose of increasing the filtration rate of the slower filtering fibers. These procedures include thermal treatment of the fiber; the application of assorted coatings to the fiber; the use of organic coagulating agents and inorganic salt solutions; and soaking the fibers in sodium silicate solutions followed by dewatering with an acid treatment to minimize the accelerating effect of sodium silicate on hydraulic cement. However, notwithstanding the disadvantages presented by the slower filtration fiber in wet machine processing, only a very few of these treatments methods have been extensively used for upgrading the filtration characteristics of chrysotile asbestos fiber.
For example, a silicate treatment of asbestos fibers increases filtration rates, but results in considerable decrease in strength of products incorporating the treated fiber when large quantities of silicate are added in an effort to greatly increase filtration rates. A preferred silicate treatment procedure, which is used but can nonetheless result in asbestos-cement products of decreased strength at high silicate levels, is described in U.S. Pat. No. 3,173,831. This method, which includes spraying a silicate solution onto asbestos fibers, greatly improves the filtration characteristics of the fiber, but has a strength-decreasing effect on asbestos-cement products. Thus, for practical purposes, it was necessary to limit the amount of silicate added to the fiber to prevent a serious decline in product strength. While this limited amount of silicate improved the filtration characteristics somewhat, further improvement was still desirable. Higher amounts of silicate would further improve filtration, but accompanying manufacturing problems and resulting strength losses make the higher amounts of silicate impractical.
It has previously been found that a nonfoaming anionic dispersant can be added to chrysotile asbestos fibers, or to a slurry or paste containing asbestos fibers, to improve the filtration characteristics of the fiber, as described in U.S. Pat. No. 3,891,498 and Canadian Pat. No. 886,051. While this treatment produces improved filtration rates, it has been found that the effect of the nonfoaming anionic dispersant is not cumulative. That is, while increased filtration rates result from addition of the dispersant up to a certain weight percentage level, once that level is reached further addition of the dispersant does not result in additional increases in the filtration rate. The maximum filtration rate achieved by the addition of the nonfoaming anionic dispersant is thus limited.
It has recently been proposed to combine the silicate treatment with the addition of a nonfoaming anionic dispersant, as disclosed in U.S. Pat. No. 3,715,230. Such a process results in an additive effect on improved filtration and, unexpectedly, does not decrease the strength of the products made from the treated fibers, as do fibers treated with the same level of an alkali metal silicate alone. In addition, this dual treatment theoretically permits the use of higher concentrations of alkali metal silicate than was previously thought to be commercially practical. However, because of the tacky nature of sodium silicate, manufacturing problems develop long before the theoretical maximum amount of sodium silicate is reached. Also, there is still a present need to increase further the filtration rate of chrysotile asbestos fiber.
It has also recently been proposed to increase the filtration rate of asbestos fibers by treating the fibers with an aqueous solution of a phosphate compound or a corresponding acid, as disclosed in U.S. Pat. No. 3,535,150. While this phosphate treatment improves the filtration characteristics of untreated fiber without significantly reducing the strength of subsequent asbestos-cement products, the filtration characteristics of the phosphate-treated fiber are still much lower than desired.